Thermal hydrodealkylation process



March 19, 1968 N. L. CARR ETAL 3,374,280

THERMAL HYDRODELKYLATION PROCESS Filed June 20,' 1966 RODA/Ey E. PETE'PS/V lifted States Patent O THERMAL HYDRGDEALKYLATIGN PROCESS Norman L. Carr, Allison Parli, and Rodney E. Peterson,

Oakmont, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed June 20, 1966, Ser. No. 558,993 Claims. (Cl. 260-672) ABSTRACT 0F THE DISCLOSURE Alkyl aromatic hydrocarbon feed stocks containing toluene are sulbjected to thermal hydrodealkylation in the presence of hydrogen and at least one alkyl benzene having at least 8 carbon atoms per molecule. The presence of C8+ alkyl benzenes increased selectivity and conversion of toluene to benzene at lower reaction temperatures and shorter reaction times.

This invention relates to a process for the thermal hydrodealkylation of alkyl aromatic compounds. More particularly, this invention relates to such a process wherein an increased reaction rate and higher benzene yields are obtained.y

Toluene can be dealkylated to benzene by subjecting it in the presence of hydrogen to an elevated temperature and elevated pressure for a controlled length of time. As a result of such reaction conditions, the methyl group is cleaved fromvthe toluene. The phenyl and methyl groups combine with hydrogen present to yield benzene and methane.

The thermal hydrodealkylation reaction is highly exothermic and has generally required the use of large reactors and severe reaction conditions.

It is an object of this invention to provide an improved thermal hydrodealkylation process.

It is another object of this invention to provide a thermal hydrodealkylation process orf alkyl aromatics which results in an increased reaction -rate and higher yields of benzene at relatively lower reaction temperatures and/ or using relatively smaller reactors.

These and other objects are attained by the practice of this invention which, briefly, comprises subjecting a gaseous mixture which consists essentially of hydrocarlbons (i.e., which is substantially free of components which contain atoms other than carbon or hydrogen) and hydrogen in a rea-ction zone to a reaction temperature of from about 1000 to 1800 F. The hydrocarbons which are employed include a mixture comprising toluene, and from about l to 50% by weight, based on the weight of the toluene, of at least one alkylbenzene having at least 8 carbon atoms. The hydrocarbon feed which is employed in the gaseous reactionmixture has a boiling point in the range of from about 231 to 450 F. After the thermal hydrodealkylation reaction is complete, the product, benzene, is recovered from the by-products comprising primarily methane and polyphenyls (mainly di-phenyl) by known techniques.

The invention will be further illustrated with reference to the accompanying drawing.

Referring to the drawing, the hydrocarbon feed stock is fed into the system by way of line 11. The feed comprises a mixture comprising toluene and at least one alkylbenzene having at least 8 canbon atoms. The alkylbenzene having at least 8 carbon atoms can be, for example, m-xylene, o-xylene, p-xylene, ethylbenzene, propylbenzene, butylbenzene and other C9 and C10 alkylbenzenes. An appreciable increase in dealkylated product selectivity and in the rate of the thermal hydrodealkylation reaction can be obtained by the inclusion in the feed of 3,374,289 Patented Mar. 19, 1968 ICC as little as 1% by weight C8 and/or C8+ alkylbenzenes, and a marked increase will be obtained by the use of about 3 to 20% of such alkylbenzenes. However, even larger proportions, as much as 50% of such alkylbenzenes can be used. No appreciable further gains are obtained by the inclusion in the feed of still larger proportions of C8 and/ or C8+ alkylbenzenes, since incremental gains in reaction rate and product selectivity diminish and hydrogen consumption increases, with such larger proportions.

Line 11 is provided with a pump 12, for compressing the feed to an elevated pressure. The compressed feed is then passed by line 13 to a heat exchanger 14 in which it is indirectly heated with hot product eliuent obtained as hereinafter described.

Makeup hydrogen-containing gas at an elevated pressure is introduced to the process through a line 15 whi-ch communicates with line 13. Hydrogen-containing recycle gas is introduced into line 15 by a line 16. The makeup hydrogen gas stream, as Well as the hydrogen-containing recycle gas stream, need not be pure hydrogen. These streams may contain between about 50 and 100% hydrogen and, preferably, between about and 85% hydrogen.

The reactant feed stream comprising toluene, C8+ hydrocarbons and hydrogen may contain a hydrogen to hydrocarbon mol ratio within the range of from about 1.5 to 20.0 and, preferably, from about 3 to 8. The reactant feed stream is heated in the heat exchanger 14 to a temperature of about 340 F. The reactant stream is then passed through a line 17 to a second indirect heat exchanger 18 wherein the reactant stream is further indirectly heated with reaction effluent to a temperature of about 940 F. The preheated reactant stream is passed from the exchanger 18 by a line 19 to a heater or furnace 20 wherein iinal heating of the reactant feed stream up to the reaction temperature is accomplished. The reactor feed stream heated to reaction temperature in the heater or furnace 20 is then passed by a line 21 to the first reactor 22. An effluent is recovered through the top of the reactor 22 by a line 23. This eflluent may optionally be quenched to a lower temperature by direct mixing with a cool hydrogen-containing recycle stream obtained from a high pressure flash drum more fully described hereinafter and introduced by a line 24. The effluent in line 23 is thereafter introduced into the top of a second reactor 25.

The thermal hydrodealkylation reaction which occurs in reactors 22 and 25 is conducted at a temperature of from about 1000 to 1800 F. and a pressure of from about to 1000 p.s.i.g. with a contact time or residence time of the reactants in the reactor of from about 1 to 600 seconds. In a preferred embodiment of this invention, the reaction is conducted at a temperature of from about 1100 to 1350" F. and a pressure of from about 400 to 600 p.s.i.g. for from about 10 to 100 seconds.

An effluent is recovered from the bottom of the reactor 25 by a line 26. The eiluent in line 26 is then quenched to a temperature below the reaction temperature by direct mixing with a portion of the cool recycle stream obtained from the high pressure flash drum hereinbefore mentioned and introduced by a line 27.

In reactor 22, the flow of the reactant feed stream is upward and in reactor 25 it is downward. As previously mentioned, the quenching of the eluent from reactor 22 is optional. Therefore, the line 24 may be omitted if a quench is not employed at this point. Moreover, quenching at this point and/or quenching of the effluent from reactor 25 may also be accomplished by means of liquids or gases other than the hydrogen-containing recycle stream illustrated; or by indirect downstream ofVV the second reactor which is used to separate the products obtained from the reactor.

The eluent in the line 26 after quenching is passed to the indirect heat exchanger 18, wherein the effluent gives up a portion of its heat to preheat the feed in the line 17, thereby cooling the effluent to a temperature of about 700 F. The partially cooled effluent is then passed by a line 28 to a reboiler 29 associated with the bottom of a fractionator, more fully discussed hereinafter, to provide the heat duty of the fractionator. That is, in reboiler 29, the etiluent gives up a portion of its heat and is cooled to a temperature of about 80 F. by indirect heat exchange with a liquid stream withdrawn from the lower portion of the fractionator. The thus cooled efliuent is passed from reboiler 29 by line 30 to reboiler 31 associated with the bottom of a product stripper tower more fully discussed hereinafter. In reboiler 31, the effluent is further cooled to a temperature of about 495 F. from which it is withdrawn and passed by line 32 to a hydrogenation chamber 59 wherein the effluent is subjected to mild hydrogenation conditions as described in copending applications, Ser. Nos. 466,255 or 466,256, filed June 23, 1965.

In chamber 59, any materials containing aliphatic unsaturation in the liquid product stream are hydrogenated to saturated products, thereby facilitating subsequent fractionation. The thus treated liquid product stream is then passed by line 60 to heat exchanger 14 wherein the eiluent gives up additional heat to the reactant feed stream in line 13, thereby being cooled to a temperature of about 345 F. Accordingly, the hot effluent recovered from reactor 25 supplies the heat duty of the fractionator and the product stripper in addition to supplying the major portion of the heat to bring the reactant feed stream up to reaction temperature.

The hot effluent is then passed from the exchanger 14 by line 33 to a suitable cooler 34. The cooler 34 may be any suitable arrangement of coolers comprising a water cooler, air cooler, or a combination thereof which will sufficiently cool the euent for passage by a line 35 to a high pressure ash drum 36 maintained at a pressure of about 400 p.s.i.g. and a temperature of about 100 F.

In the high pressure ash drum 36, a vaporous stream comprising hydrogen, methane and a small amount of entrained benzene product is separated from a major benzene liquid product stream. The vaporous stream is removed from the drum 36 by line 37 and separated into two streams with the major portion thereof being passed by line 38y to a recycle compressor 39 and the minor portion of the stream being passed for further treatment by line 40 as discussed hereinafter.

The recycle gas stream is compressed in the compressor 39 to an elevated pressure of about 525 p.s.i.g. suitable for recycle to the reactors, thereby raising the temperature of this stream to about 130 F. The thus compressed recycle stream is passed by line 16 to lines 27 and, optionally, 24 for use as quench material in the reactor eiuent streams as discussed above. A third portion of this recycle stream is combined with hydrogenrich makeup gas introduced to the process by line 15 and is thereafter combined with the hydrocarbon feed to be demethylated prior to the heat exchange steps hereinbefore discussed.

The vaporous stream of minor portion in line 40 recovered from the high pressure ash drum is further treated to obtain maximum recovery of entrained benzene product material. To accomplish this end, the vaporous stream in line 40 is passed to an indirect heat exchanger 41 wherein it is cooled to a temperature of about 65 F. by indirect heat exchange with refrigeration flash vapors obtained as hereinafter described. The vaporous stream cooled in the indirect heat exchanger 41 is then passed by line 42 through a refrigeration cooler 43 to further cool the vaporous stream to a temperature of about 40 F. The thus cooled vaporous stream is then passed by a line 44 to a separator drum 45 maintained at a temperature of about F. and a pressure of about 380 p.s.i.g.

In separator drum 4S, a vapor stream, referred to herein as refrigeration ash vapors, is separated and recovered from a liquid benzene stream. The refrigeration flash vapors of reduced temperature are passed by line 46 to the heat exchanger 41 to precool the vaporous stream in line 40 as described above. The refrigeration flash vapors are recovered from heat exchanger 41 by line 47 and passed to the hydrogen plant.

The liquid benzene stream separated in drum is withdrawn and passed by line 48 to line 49 wherein it is combined with the liquid stream recovered from the high pressure separator drum 36. The thus combined stream is then passed to the upper portion of a product stripper tower 50.

The stripper tower 50 is maintained at a temperature in the range of from about 110 F. to about 450 F. and a pressure in the range of from about 305 p.s.i.g. to about 310 p.'s.i.g. with heat being supplied to the lower portion of the stripper tower by passing a liquid stream withdrawn from the lower portion thereof by line 51 to heat exchanger 31 and thereafter returning the heated withdrawn stream to the tower by line 52 to supply the heat duty of the stripper tower In stripper tower 50 a vaporous stream comprising about 2 mol percent benzene is recovered from the liquid product introduced thereto by line 49 and removed from the upper portion of the tower by line 53. The vaporous stream in line 53 is passed through refrigeration drum 43 to lcool this stream to about 40 F. from whence it is withdrawn and passed by line 54 to separator drum 55 maintained at a temperature of about 40 F. and a pressure of about 290 p.s.i.g. In separator 55 a vapor stream is separated from a liquid stream comprising benzene, the vaporous stream is removed therefrom by line 56 and the liquid stream Ais removed therefrom by line 57.

A stripped liquid product stream comprising benzene is recovered from the bottom of the stripper tower 50 by a line 58 and is passed to fractionator 62. The liquid stream in line 57 recovered from separator 55 is also connected to line `61 in order that this recovered liquid material may be passed to fractionator 62.

Fractionator -62 is maintained at a temperature in the range of from about 210 F. to about 422 F. The heat duty of the fractionator 62 is supplied by withdrawing a liquid stream from the lower portion of the tower by line 63, passing the thus withdrawn material through reboiler 29 and returning the heated liquid stream to the tower by line 64. Provision may also be made for recycling a portion of the liquid bottoms withdrawn from the bottom of the fractionator 62 to reboiler 29 where it is heated and thereafter returned to fractionator 62. A portion of the liquid bottoms may also be recycled for use as a quenching liquid for the effluent from reactor 22 and/or the effluent from reactor 25. The remaining portion of the bottoms from fractionator 62 is withdrawn from the process by line 65 for further use or treatment as desired.

Fractionator tower 62 is designed to withdraw a benzene product stream -from the upper portion thereof by line 66 which is provided with cooler 67 for cooling the benzene product stream to a temperature of about F. To assure recovery of a high purity 'benzene product stream from the fractionator, the benzene stream is withdrawn from the fractionator at about the fth tray and any lower boiling materials are withdrawn from the top of the tower by line 68, cooled in cooler 69 to a temperature of about F. and then passed to separator drum 70. A portion of this material is employed as a cool reflux stream and is withdrawn from the separator 70 and returned to the top portion of the fractionator above the point of withdrawal of benzene product materal by line 71.- Line 72, 'which is connected to line 71,

is provided for withdrawing any excess reflux material from the system.

Unconverted toluene and C8+ alkylbenzenes are removed from a lower portion of the fractionator by line 73 provided with cooler 74 for reducing the temperature of the stream to about 100 F. The cooled stream may be recycled by line 75 to the feed in line 11.

As a safety factor in the refrigeration section of the process herein described, provision is made for introducing a portion of the toluene and Cai hydrocarbon feed, when necessary, to the vaporous streams in lines 40 and 53 by w'ays o-f lines 76 and 77 to avoid freezing of ahy benzene material, cooled in refrigeration exchanger 43.

The practice of this invention results in an increased rate of the thermal hydrodealkylation reaction and permits the use of lower reaction temperatures and/or smaller reactors since contact time requirements are substantially reduced. Higher yields of benzene are obtained. Moreover, there is an improvement in benzene selectivity since there is an increase in the rate of benzene formation without a corresponding increase in the rates of hydrogenolysis or diphenyl formation. These advantages 'are obtainable by adjusting the feed fractionation to leaves the desired amounts of C8 and/or C8* alkylbenzenes in the feed, and without the additional cost or complexity of adding -any extraneous material, without increased hydrogen consumption that is unproductive of additional benzene product, and without formation of product requiring different separation facilities.

The addition of Cgi alkylbenzenes to toluene -feed in a thermal hydrodealkylation process creates a synergistic effect and results in an unusually high over-all reaction rate constant, i.e., the reaction rate constant is increased by a factor of about four.

The following example illustrates this invention:

Example This example employs the process and apparatus illustrated in the drawing and described hereinabove. In one run, a hydrocarbon feed comprising 5l mol percent of toluene, 45.5 mol percent of mixed xylenes and ethylbenzene and 3 mol percent of C9 alkylbenzenes is passed to a thermal hydrodealkylation reactor at a rate of 1200 barrels per day. The toluene and the C8 and C9 alkylbenzene components of the hydrocarbon feed were obtained by fractional distillation of a crude aromatic extract obtained by solvent treatment of the Udex process of a naphtba refonnate product. The distribution of C8 alkylbenzene products obtained in this way is known to lbe about 30 percent of each of the In, o and p xylene isomers and about l0 percent ethylbenzene. Hydrogen is introduced into the reactor in an 'amount sufficient to give the hydrogen to hydrocarbon mol ratio specified in the table. The thermal hydrodealkylation reaction is conducted at a pressure of 475 p.s.i.g. and at the average temperature specified in the table. The residence time in the reactor is 49 seconds.

`For the purpose of comparison, t-wo runs are conducted in which a hydrocarbon yfeed stream comprising 100% toluene is passed to a thermal hydrodealkylation reactor at a rate of 1290 barrels per day. Hydrogen is introduced into the reactor in an amount suflicient to give the hydrogen to hydrocarbon rnol ratios specified in the table. The reactions are conducted at a pressure of 475 p.s.i.g. and at the average temperatures specified in the table. The residence time in `Run 2 is `64 seconds, and in Run 3 is 55 seconds.

6 For each of the of the three runs described above, the percent of aromatic conversion and the benzene selectivity (expressed in mol percent) lare determined and are set -forth in the following table.

The rate constant for the reaction CqH8- C6H6 in Run l is calculated to be four times the rate constant for the same reaction in Runs 2 and 3. From a comparison of the results obtained in the various runs, it will be seen that a higher conversion, coupled with a high benzene selectivity, were obtained at a lower reaction temperature and in a shorter reaction time when .the C84' alky-lbenzenes were present. A lower operating temperature is advantageous from the standpoint of lower operating costs and a shorter reaction time is advantageous from the standpoint of the lower investment cost for the correspondingly smaller reactor volume.

Obviously, many modifications .and variations of the invention as hereinabove set forth can be made without departing from the spirit and scope thereof; and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A process for the thermal hydrodealkylation of toluene which comprises subjecting a substantially par- 'an-free gaseous mixture consisting essentially of hydrocarbons and hydrogen in a reaction zone to a reaction temperature of from about 1000 to l800 F., said hydrocarbons comprising a mixture of toluene and from about l to percent by weight, based on the weight of the toluene, of at least one alkylbenzene having at least 8 carbon atoms and having a boiling point in the range of from about 231 to 450 F., and thereafter recovering the hydrodealkylated product.

2. A process as defined in claim 1 wherein said thermal hydrodealkylation reaction is conducted at a pressure of from about to .1000 p.s.i.g. for from about 1 to 600 seconds and the hydrogen to hydrocarbon mol ratio is within the range of from about 1.5 to 20.0.

3. A process as defined in claim 1 wherein said thermal hy-drodealkylation reaction is conducted at a temperature of from about 11100 to 1350* and a pressure of from about 400 to 600 p.s.i.g. for from about 10 to 100 seconds and the hydrogen to hydrocarbon mol ratio is with-in the range of from about 3 to 8.

4. The process of claim 1 wherein said a'lkylbenzene contains from 8 to l0 carbon atoms.

5. The process of claim 1 wherein said alkyl benzene contains 10 carbon atoms.

References Cited UNITED STATES PATENTS o DELBERT E. GANTZ, Primary Examiner.

G. E. SCHMITKONS, Assistant Examiner. 

